Helicopters typically have two engines that are connected through a combiner transmission to share the load of the rotor. It is desirable to share the load equally between the two engines so that the engines are more likely to deteriorate at the same (or similar) pace, and impart less stress to the combiner transmission. Helicopter engine controllers are typically configured to selectively implement one of a plurality load sharing control methods, and control logic that selects the control method. These control methods may include, for example, torque matching, a temperature matching, and a speed matching. With the torque matching method, measured engine torque is equalized, with the temperature matching method, measured engine temperatures are equalized, and with the speed matching method, measured engine speeds are equalized.
Unfortunately, none of the above-described control methods can continuously produce identical or synchronized performance margins for the two engines. Performance margin is an engine condition indicator and, as is generally known, is defined as the difference between one or more performance parameters at rated power and the corresponding limits of the performance parameters. As may be readily understood, because performance margin is measured at max rated power, two engines can have very different performance margins even if the engines have similar performance characteristics at lower power. Moreover, each engine will typically exhibit its own unique performance deterioration characteristics.
When the performance margin of an engine reaches zero, the engine is removed from the aircraft for repair, overhaul or replacement. Significant performance margin differences occur when a new or overhauled engine is installed with an engine that has already lost some performance margin. Thus, it is desirable to match the performance margins of both engines so that the engines can be simultaneously removed. However, the commonly used load sharing methods mentioned above do not ensure that the performance margins are matched. In particular, torque matching tends to cause the engine with a lower temperature margin to run hotter and increase the temperature margin split between the two engines. Temperature matching at part power does not guarantee that the temperature margins match at max rated power since the engines may have differently shaped temperature vs. torque characteristic curves. And speed matching at part power does not guarantee that the speed margins match at max rated power since the engines may have differently shaped speed vs. torque characteristic curves.
When the performance margins of two engines are not matched, this can lead to reduced engine life, reduced aircraft availability, and increased maintenance costs. Moreover, helicopter engine controls are typically configured such that a pilot may manually select the control method to be used in order to attain maximum power from both engines. This can lead to increased pilot workload. For example, if one engine reaches the temperature margin limit before the other engine while operating in the torque matching mode, the pilot will need to switch to the temperature matching mode to allow the other engine to attain its maximum power.
Hence, there is a need for a system and method of matching the performance margins of two engines. In doing so, the system and method will provide increased engine life, increased aircraft availability, reduced maintenance costs, and reduced pilot workload. The present invention addresses this need.